Naval Research Lab testing use of solar photovoltaics in groups of UAVs

WASHINGTON. Researchers at the U.S. Naval Research Laboratory (NRL) Vehicle Research Section and Photovoltaic Section are making progress on their investigations into the presence of solar photovoltaics (PV) to the proven concept of autonomous cooperative soaring of unmanned aerial vehicles (UAVs). These cooperative autonomous soaring techniques will enable long-endurance flights of unmanned solar-powered sailplanes that can support a 24/7 information, surveillance, and reconnaissance (ISR) mission.

The ability to send several UAVs aloft at once, sharing data, will benefit the warfighter because it will reduce the amount of batteries or fuel they must carry into the field and

Holding the photovoltaic (PV) UAV based on the SBXC sailplane are Dan Edwards (left) and Trent Young (right), two members of the NRL's "Solar-Soaring" research flight crew. (Photo credit: U.S. Naval Research Laboratory.)

Dr. Dan Edwards, aerospace engineer and principal investigator for the solar-soaring program, said, "NRL has twice flown our solar UAV [based on the SBXC sailplane] over 10 hours using a combination of solar photovoltaics and autonomous soaring as part of the ‘solar-soaring’ research program. This research is investigating the value of combining autonomous soaring algorithms and solar photovoltaics for capturing energy from the environment to extend flight endurance and mission operations of an aircraft.”

Researchers integrated a photovoltaic array, custom built in NRL's Vehicle Research Section and Photovoltaic Section, into the center wing panel of the PV-SBXC aircraft as a drop-in replacement to the original wing. A power management and distribution system (made by Packet Digital, Inc.) converted the power from the solar arrays into direct current (DC) voltage. The aircraft's electric motor can use the DC voltage either for propulsion or to recharge a "smart battery." The NRL also incorporated an autonomous-soaring software algorithm, which would ordinarily monitor the local vertical winds around the aircraft, to direct the UAV to orbit in any nearby updrafts, very similar to soaring birds. For testing purposes, however, the NRL disabled the algorithm in order to assess the crafts' solar-only performance. Passive soaring (which means that meaning no specific maneuvers are attempted to catch thermals) was still allowed, to let the aircraft turn the motor off if altitude increased because of an updraft along the aircraft’s predefined flight path.

“The experiments confirm significant endurance gains are possible by leveraging thermal updrafts and incident solar radiation, rather than ignoring these free sources of energy,” stated Dr. Edwards. “Future testing will focus on quantifying the trade space between improvements in solar-cell efficiency and combining with autonomous soaring for improved solar recharging.”